The deal was first announced in July last year, and was billed as “a merger of equals”.

However, in a joint statement issued late last night, the two companies said the merger process had been terminated “in view of difficulties in completing the transaction within a reasonable timeframe”. No termination fees will be paid by either party.

Omnicom owns several research agencies, including Communispace, Flamingo and Hall & Partners.

Maurice Lévy, Publicis’s chairman and CEO, and John Wren, president and CEO of Omnicom Group, said: “The challenges that still remained to be overcome, in addition to the slow pace of progress, created a level of uncertainty detrimental to the interests of both groups and their employees, clients and shareholders. We have thus jointly decided to proceed along our independent paths. We, of course, remain competitors, but maintain a great respect for one another.”